function [ Ak , Bk , Fk ] = Coeff(X,a)

  global m S CD g rou0 Hre RE

  v   =    X(1) ;
  the =    X(2) ;
  x   =    X(3) ;
  y   =    X(4) ;

  col = a ;
  
  rou =    rou0 ;
  aa  =    - rou * S * CD / m / 2 ;
  
  dv  = - rou * v^2 * S * CD / 2 / m - g * sin( the ) ;
  
dthe  = ( col - g * cos( the ) ) / v ;

  dx  =   v * cos( the ) ;
  
  dy  =   v * sin( the ) ;

  Fk  = [ dv ; dthe ; dx ; dy ] ;
  
  Ak  =  [  aa * v * 2   , - g * cos(the) , 0 , 0  ;
           (g * cos(the) - a) / v^2 , g*sin(the)/v,0,0 ;
            cos(the) , -v*sin(the) ,0 ,0 ;
            sin(the) ,  v*cos(the) ,0 ,0 ] ;
           
  Bk  =  [ 0 ; 1/v ; 0 ; 0 ] ;
  
end